Error Detection Algorithms, Compression, Multiplexing, (ch. 13)

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Error Detection Algorithms, Compression, Multiplexing, (ch. 13)

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Error Detection Algorithms, Compression, Multiplexing, (ch. 13)

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Error Detection Algorithms, Compression, Multiplexing,(ch. 13)

ENGR 475 – Telecommunications

September 12, 2006

Harding University

Jonathan White

- Hopefully built into the physical layer encoding
- For example, Differential Manchester physical encoding can detect errors in 1 bit

- Error Detection is also employed in other layers further up the protocol stack.
- We are going to be looking at transport/network layer error detection.
- Your application will also hopefully do error detection.

- 3 main low-level methods:
- Parity Bit (even or odd)
- VRC, often used in trusted mediums
- Detects approximately 65% of errors

- Longitudinal redundancy check
- LRC, rarely used
- Detects 85% of errors

- Cyclic redundancy check
- CRC, very awesome, will use all your math skills
- Detects upwards of 99.99995% of all errors

- Parity Bit (even or odd)

- Add a single parity bit to the end of each byte sent.
- Can be even or odd parity.
- For odd parity, the number of 1’s must add to an odd number, including the parity.
- For even parity, the number of 1’s must add to an even number, including the parity.
- Used inside of computers.
- What’s the overhead for parity bits?
- What type of errors aren’t found?

- Adds horizontal and vertical checking.
- Works on an n-1 X n grid.
- In our example, 8 vertical 7 bit words.

- Better at detecting errors, but only slightly.
- What’s the overhead in our example?
- Can you make an error that’s not detected?

- Type of hash function
- Works on a block of data
- Adds extra bits to the end of the data using binary division by relatively prime divisors.
- Similar to previous

- The receiver then computes the same hash function on the data.
- If the CRCs match, then the data is trusted to be what was sent
- Note: Some messages can get the same hash value, but this is a very low percentage.

- If the CRCs match, then the data is trusted to be what was sent

- CRCs are one way
- Can be very many bits long
- Can be very fast
- Examples of common hash functions:
- MD5
- SHA1
- RIPEMD

- Used on almost every digital communication link
- http://en.wikipedia.org/wiki/Cyclic_Redundancy_Check

- RFID

- Lossless vs Lossy
- Where would each work?
- Video (VGA SVGA), voice, movies, source code, mathematical formulas

- Where would each work?
- Digital voice is compressed only rarely.
- Takes a lot of effort to compress and voice is very time sensitive.

- WinZip and File Structures class.

- Definition:
- The process of combining two or more communication paths into one path.

- Why use multiplexing:
- Saves communication paths, which saves money.

- 3 main types:
- Time division
- Frequency division
- Space Division (not really multiplexing)

- Only digital
- The multiplexing unit slots time.
- Each of the sends is assigned one slot to send in.
- If the multiplexing unit can slot time fast enough, the senders will each get a guaranteed certain rate of speed.
- The limit is on the bandwidth of the downward pipe.
- That’s why we use fiber.

- Slotting time is a big job. This is actually a slow process.
- Statistical multiplexing.

- Invented in WW2.
- Used in T1s by the 1960s.

- Used in:
- T1 architecture of telephones
- WAV format
- GSM cell phone network (TDMA).

- Negatives:
- Too slow

- An analog technology.
- Combines several signals by onto 1 medium be sending signals in defined frequency ranges.
- The receiver can then “tune” into that frequency.
- Very easy to do.
- However, it takes up more bandwidth than TDM.

- Used in cable TV, FM/AM radio, portable telephones.
- http://en.wikipedia.org/wiki/Frequency_division_multiplexing

- Not really multiplexing
- Each signal has its own wire that is just bundled together.
- This is what the telephone company does at a junction box.